The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro Weight Loss: Side Effects & NHS Prescribing Guide
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection licensed in the UK for adults with obesity or overweight. Clinical trials showed participants lost an average of 15–21% of their body weight, though side effects and individual results vary significantly. This guide covers what to expect, NHS eligibility, and how to access this medication through UK healthcare services.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
NHS Eligibility and What Clinical Trials Revealed
In the UK, Mounjaro is available through NHS specialist weight management services (tier 3) and private online prescribing. NHS eligibility requires a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds apply due to higher health risks at lower BMI levels.
The SURMOUNT-1 clinical trial demonstrated weight loss results: participants on 5mg lost approximately 15% of body weight, 10mg achieved about 19.5%, and 15mg (the maximum dose) showed around 21% average loss over 72 weeks. These results were achieved alongside a reduced-calorie diet and increased physical activity—medication alone does not clinically studied weight loss. Individual responses vary considerably based on genetics, adherence, and lifestyle factors. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses.
Clinically BackedWeight Loss
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Side Effects: What's Common and When to Seek Help
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain—most are mild to moderate and often improve as your body adjusts. Common side effects affecting up to 1 in 10 include indigestion, acid reflux, fatigue, dizziness, injection site reactions, and hair loss (alopecia). These typically reduce after the first few weeks as your body adapts to the medication.
Seek immediate medical attention for severe, persistent stomach pain spreading to the back (possible pancreatitis), serious allergic reactions, or gallbladder symptoms. Always inform your prescriber of all medications you take, as Mounjaro may affect absorption of oral contraceptives and diabetes medications. Your GP or prescriber can discuss side effect management and whether Mounjaro is right for you.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
How much weight will I lose with Mounjaro?
Clinical trials showed an average loss of 15–21% of body weight over 72 weeks on maximum doses, but individual results vary significantly. Some people lose more, others less. Weight loss depends on your starting weight, diet adherence, physical activity, and how your body responds to the medication. Use our free GLP-1 weight loss calculator to see a personalised projection.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP + GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1-only agonist. Clinical trials showed Mounjaro achieved slightly higher average weight loss (21% vs 15% on Wegovy), but both are effective. Your prescriber will assess which is more suitable based on your health profile and medical history.
Can I get Mounjaro on the NHS in the UK?
Yes—Mounjaro is available through NHS specialist tier 3 weight management services following NICE approval. However, NHS access varies significantly by region and waiting lists can be lengthy (many months to over a year). Private online prescribing through GPhC-registered pharmacies offers faster access but involves private costs (typically £129–£320/month depending on dose).
What happens if I stop taking Mounjaro?
Weight regain is common after stopping—clinical trials showed most people regained weight within months without ongoing treatment. Mounjaro works best as part of long-term weight management combined with sustainable diet and exercise changes. Discuss duration and stopping plans with your GP or prescriber before starting treatment.
Are Mounjaro side effects permanent?
Most side effects (nausea, diarrhoea, constipation) are temporary and improve within 2–4 weeks as your body adjusts. However, some people experience persistent side effects throughout treatment. Hair loss and appetite changes may take longer to resolve. If side effects are severe or persist, contact your prescriber—dose adjustment or alternative medications may be options.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.